The present invention relates to systems and methods for heating a moving liquid or slurry using microwave energy. More particularly, the invention provides uniform heating throughout the desired heating volume by applying higher order resonance modes in a cylindrical wave-guide.
A variety of food processing and other industrial processes require continuous heating of a moving liquid or slurry. This heating was once performed, for example, by using steam or hot water jackets to surround pipes carrying the fluid of interest, or by using heat exchangers. More recently, microwave heating has been employed to provide the required heating for these processes.
One example of microwave heating is provided in U.S. Pat. No. 5,697,291 to Burgener et al. This patent describes a continuous flow thermal pasteurization and enzyme inactivation method and apparatus for economically and precisely raising the temperature of a flowing fluid to a point at which bacterial and enzymes are inactivated. This method and apparatus involve two stages. In a first preheat stage, the fluid is preheated to within several degrees of the pasteurization or inactivation temperature using heat regenerated from the pasteurization or inactivation product, or by using heat provided by surface conductance from a heated vapor, heated liquid or a heated element. In the second or final of the two heating stages, the preheated fluid is gradually heated with microwave heating to the pasteurization or inactivation temperature for precisely and evenly controlling the temperature of the fluid. Preferably, the microwaves are applied to the fluid through the forced absorption of energy over substantially long lengths of product tubing.
In another example of microwave heating of a moving fluid in an industrial process, U.S. Pat. No. 5,719,380 to Coopes et al. provides an apparatus for heating mixtures in the manufacture of photographic dispersions. In this apparatus, a chamber for receiving microwave energy input is provided in the form of a section of rectangular wave-guide where the wave-guide is dimensioned to propagate an input of microwave energy in the TE10 field mode. The wave-guide section is terminated by a short circuiting metal plate, which sets up a standing electromagnetic wave inside the wave-guide. A straight length of microwave transparent tubing passes transversely through the wave-guide and the fluid to be heated is passed through the tubing.
For many industrial heating processes, the solutions described above are not sufficient. In particular, many industrial heating processes require rapid heating with good uniformity (to prevent, for example, localized boiling) throughout a large volume. This can be a particularly challenging problem when heating a heterogeneous solution such as a slurry, or a fluid flowing through or over a catalyst.
The present invention addresses the problems in the prior art by providing a microwave applicator capable of uniformly heating large volumes of fluid or heterogeneous fluid solid combinations while minimizing hot spots that can cause localized boiling. In one aspect, the invention provides a microwave applicator for heating a moving fluid. The applicator includes a heating chamber having a fluid inlet and a fluid outlet and through which the fluid to be heated flows. The applicator also includes a microwave energy source and a microwave circuit having at least one wave-guide element. The microwave circuit transforms microwave energy from the microwave source into a cylindrical wave-guide mode within the heating chamber for uniformly heating fluid flowing through the heating chamber. In a further aspect of the invention, this technology is applied as a method for applying microwave energy for heating a moving fluid. This method includes passing a fluid from a fluid inlet, through a heating chamber, and out a fluid outlet; and applying a microwave energy source through a microwave circuit including at least one wave-guide element to transform microwave energy from the microwave energy source into a cylindrical wave-guide mode within the heating chamber to uniformly heat the fluid flowing through the heating chamber.
In specific embodiments of the invention, the microwave circuit transforms microwave energy from the energy source into a cylindrical wave-guide mode that is higher than the dominant mode. In separate embodiments, the microwave circuit transforms microwave energy from the microwave energy source into the TE21 cylindrical wave-guide mode and into the TM11 cylindrical wave-guide mode, respectively. In addition, the microwave circuit can be configured to transform a majority of the microwave energy into a single wave-guide mode that is higher than the dominant mode, and in a more specific embodiment, can be configured to transform substantially all of the microwave energy into a single wave-guide mode that is higher than the dominant mode.
To achieve the desired transformations and excite a cylindrical wave mode, the microwave circuit can include an rf match cavity, the rf match cavity being a cylindrical chamber surrounding the heating chamber. The rf match cavity can also include two input ports for receiving microwave energy via the microwave circuit. In order to provide energy to two ports, the microwave circuit can further include a three port signal divider, a first wave-guide element extending between the microwave energy source and a first port of the three port signal divider, a second wave-guide element extending between a second port of the three port of the three port signal divider and a first input port of the rf match cavity, and a third wave-guide element extending between a third port of the three port signal divider and a second input port of the rf match cavity. In a specific embodiment used to excite the TE21 cylindrical wave mode, the three port signal divider is a T-coupler directing microwave energy out through the second and third ports wherein the microwave energy at one of the second and third ports is 180xc2x0 out of phase with microwave energy at the other of the second and third ports.
In further specific embodiments of the invention, safety is enhanced by providing a region surrounding the heating chamber with a pressurized gas. To provide such a region, the microwave circuit can include a dielectric window that maintains pressure surrounding the heating chamber by allowing microwave energy to pass while preventing the gas from passing through the window. The microwave circuit can further include a full height to half height transition leading into the pressure window so that the pressure window has a reduced surface area.
To aid in applying the invention to heating fluids passing through catalyst material, the heating chamber can include at least one catalyst support screen to maintain a catalyst material within the heating chamber. Even under these circumstances, heating applicators of the invention provide uniform heating throughout a mixture of catalyst material and a moving absorptive fluid having different dielectric constants.